Although most would refer to this as a Midas due to the brass reservoir, I have come across them referred to as a Huntsman Low - hence the HL prefix, as to what it means, I am not exactly sure but possibly relates to Don Lowndes who developed some early prototype pneumatic rifles adopted by Daystate.

My guess this would be early to mid 1980's in origin, as to value I had one in .22 that I bought for £175 although sold it on as I got into collecting classic FT rifles. Although one of these in .177 would interest me as they were used by a few early FT shooters, the young Mickey Andrews being one from that time period.

As stated elsewhere Sniper22 will probably be able to add more definitive info.